This invention relates generally to integrated optical circuits and the components making up such circuits. More particularly this invention relates to a type of integrated photodetector for detecting infrared signals propagating in a silicon waveguide.
The recent development of low loss single mode optical fibers with low dispersion at the 1.3 and 1.6 micron wavelengths has focused attention on long wave integrated optical circuits and optical systems that couple to such fibers. Such optical circuits and systems are useful in telecommunication, data communication, optical signal processing, optical interconnection, optical sensing, and microwave antenna control application. Semiconductor guided wave circuits are of special interest because they can, in principle, provide electrooptic integration; that is, the monolithic integration of optical guided wave components with electronic circuits and electro-optical components on a single chip.
The fundamental building blocks of such guided wave circuits are the channel waveguides which are used to make various optical components including switches, modulators and interconnects. In all these components, it is essential to keep optical propagation losses at a minimum (less than 1db per cm) in order to allow multiple guided wave components to be cascaded on one wafer (such as a switching network) without incurring a significant loss penalty.
Waveguides suitable for use in an integrated optical circuit due to their high efficiency, small size and ease of fabrication are described in a related application entitled "Method of Fabricating Low Loss Crystalline Silicon Waveguides" (Joseph P. Lorenzo and Richard A. Soref,) Ser. No. 928,349 filed Nov. 10, 1986. The methods described therein provide the desired techniques for fabricating waveguides operative at 1.3 microns to 1.6 micron wavelengths which do not suffer from the complexity or expense of using binary, ternary or quatenary alloy compositions of various materials to fabricate waveguides.
In another related application, entitled "Electro-Optical Silicon Devices" (Joseph P. Lorenzo and Richard A. Soref) Ser. No. 036,822 filed Mar. 26, 1987 various silicon devices compatible with the above-mentioned waveguides have been described. Switching and light modulating devices can be suitably integrated, manufactured and networked with low loss silicon waveguides. In spite of the above, there remains the problem of integrating passive and active low loss optical circuitry with a viable photodetector.
Schottky barrier infrared photodetectors are well known in the art. Examples of prior art Schottky barrier infrared photodetectors can be found in U.S. Pat. No. 4,531,055 to Shepperd Jr. et al and in U.S. Pat. No. 4,533,933 to Pellegrini et al. Yet a further example of a prior art Schottky barrier photodetectors is found in U.S. Pat. No. 4,467,340 to Rode et al. In all of the above referenced patents, the Schottky barrier photodetectors are positioned at a focal plane for the sensing of an external light source.
Use of these prior art detectors in an integrated optical circuit has several disadvantages. All of the above referenced photodetectors require the normal incidence of light on the sensing element and it is hard to imagine how to successfully integrate such elements into an integrated optical circuit where light signals are maintained within solid state optical channels. Going from a guided to an unguided condition is undesirable in solid state waveguide systems. Overcoming these problems increases manufacturing expenses and decreases reliability.
It therefore appears that the prior art Schottky barrier photodetectors are fundamentally inefficient for use with newly devised optical waveguide structures. Further, these photodetectors are also difficult to incorporate physically into an integrated optical circuit due to their size and material requirements.
A need therefore exists for a photodetector suitable for integration with integrated optical circuits and solid state waveguide channels. Further, a need exists for a photodetector that can be constructed of materials compatible with common optical structures made of common materials such as silicon. It would also be advantageous if such a detector offered the flexibility of a variety of possible structures to allow easy incorporation of detectors into a variety of integrated optical circuits.